Hydrocarbon wells usually comprise an exterior steel casing, which prevents the bore from collapsing, and an interior pipe or “production tube”, which conveys the natural gas or petroleum to the surface of the well. The production tube is suspended within the casing by a collar that connects the top of the production tube to the top of the casing. The collar positions the production tube concentrically within the casing so that an annular gap is formed between the exterior of the production tube and the interior of the casing.
Over the life-span of a hydrocarbon well, the gradual reduction in well pressure causes a corresponding reduction in the exit velocity of the natural resource from the well through the production tube. In addition to reducing the productivity of the well, a reduction in the exit velocity below a critical value permits vaporized acids within natural gas to condense on the interior surface of the production tube.
After the exit velocity drops below an acceptable level, production from the well is boosted by inserting a reduced-diameter, co-axial velocity string within the production tube. Over the course of time, several additional reduced-diameter velocity strings may be installed until the well is tapped out.
Due to the highly-corrosive nature of oil and natural gas, and the inherently harsh subterranean conditions deep within the well, velocity strings must be made of a material having high corrosion resistance. Due to the high pressure of the fluids contained in the well, and the excessive weight of extreme lengths of the velocity string, the velocity string must also be made of a material having high strength. Therefore, it would be desirable to provide a velocity string having good corrosion resistance and good tensile and radial strength.
It is known to make velocity strings from high-strength carbon steel, such as AISI A606 and 4130. However, high-strength carbon steel offers relatively low corrosion resistance to hydrocarbons and subterranean environments. Over time, corrosion not only adversely effects the structural integrity of the velocity string, but also adversely effects the coefficient of fluid flow friction through the velocity string. For example, rust and scale formation on the interior of the velocity string increase fluid flow friction losses through the string. As a result, high-strength steel velocity strings must be replaced in as little as 9–12 months from installation. Therefore, it would be desirable to increase production from a hydrocarbon well by reducing corrosion and scale formation on the interior of the velocity string.
In a hydrocarbon well, tar, asphalt and other impurities have a tendency to adhere to the interior of the velocity string. Over time, the accumulation of such impurities also adversely effect the coefficient of fluid flow friction through the string. Therefore, it would also be desirable to increase production from a hydrocarbon well by reducing accumulation of tar, asphalt and other hydrocarbon impurities on the interior of the velocity string.
As natural gas cools during transport through the string to the surface, the gas condenses and forms caustic condensation acids, which corrode the velocity string and increase well back pressure. Therefore, it would also be desirable to increase production from a hydrocarbon well by reducing reduce heat loss through the velocity string so that natural gas condensation is also reduced.
Common steel velocity strings are also very heavy and require the use of expensive, special equipment during installation. For example, a high tonnage crane is often needed to lift the steel supply coil, which may weigh in excess of 20 tons. At off-shore wells, specialized barges are needed to carry to the rig both the steel supply coil and a high tonnage crane. Therefore, it would be desirable to increase productivity from a hydrocarbon well by providing a velocity string which is lighter and less costly to install than a steel velocity string.